1. Field of the Invention
The present invention relates to an diagnostic apparatus and method for performing a failure diagnosis on an air-fuel ratio sensor that is used for feedback-controlling the air-fuel ratio of an internal combustion engine.
2. Description of the Related Art
To detect an air-fuel ratio of an internal combustion engine and control a fuel supply amount or the like by feeding back the detected air-fuel ratio, an exhaust pipe is provided with an O2 sensor as an air-fuel ratio sensor for detecting an O2 concentration of exhaust gas. Vehicles are equipped with a failure diagnostic apparatus for detecting a possible failure in the air-fuel ratio sensor on the basis of its output voltage. The O2 sensor is disadvantageous in that it is difficult to discriminate between a disconnection of a signal line and a ground fault at the occurrence of a failure because the internal resistance of the O2 sensor is very high and its output voltage is low until it heats and reaches an active state and its output voltage is small in a lean state even after its activation. In view of this, various techniques have been proposed to detect a possible failure in an air-fuel ratio sensor (O2 sensor).
JP-A-2002-349329 (pages 3 and 4 and FIGS. 1-3) discloses a failure diagnostic apparatus that continuously judges whether a disconnection state or a ground fault has occurred. An activation state of an air-fuel ratio sensor is judged. When the air-fuel ratio sensor is inactive, a voltage is measured after switching the input resistance of an input circuit for signal input from the air-fuel ratio sensor to an ECU.
JP-A-5-107299 (pages 3-5 and FIGS. 3 and 4) discloses a technique that ground-side voltages of air-fuel ratio sensors disposed before and behind a catalyst are offset by a prescribed value to the ground voltage and the offset-added sensor output voltages are measured, whereby a possible disconnection or short-circuiting is detected continuously while the air-fuel ratio sensors are active without changing the composition of an air-fuel mixture.
JP-A-05-223776 (pages 3-5 and FIGS. 3 and 4) discloses a technique that high-potential-side of air-fuel ratio sensors disposed before and behind a catalyst is offset (increased) to a prescribed potential and low-potential-side signals are input to a microprocessor via respective amplifiers and A/D converters, whereby various kinds of possible trouble such as a disconnection and short-circuiting to the ground or a battery of a sensor connection circuit are detected continuously while the air-fuel ratio sensors are active without changing the composition of an air-fuel mixture.
Among the above techniques for detecting a possible failure in an air-fuel ratio sensor, the technique of JP-A-2002-349329 has problems that it enables failure detection only while the air-fuel ratio sensors are inactive (i.e., failure detection cannot be performed unless the air-fuel ratio sensor is in an inactive state) and that it cannot perform failure detection continuously. The failure diagnostic apparatus of JP-A-5-107299 has problems that it is difficult to discriminate between a disconnection of a signal line of the air-fuel ratio sensors and a ground fault, that is, a type of failure cannot be judged correctly. Although the technique of JP-A-05-223776 enables a failure judgment to be performed correctly and continuously, it requires a circuit for outputting the difference between an offset-added high-potential-side output voltage and a low-potential-side output voltage of each air-fuel ratio sensor and a parallel circuit that depends on the characteristic of the air-fuel ratio sensors. As a result, a failure detection circuit that is part of a system is complicated and hence is necessarily expensive.